RSS

Email

2,317

4,689

Lightning Protection for New-generational Small Cell Infrastructure

By A. J. “Tony” Surtrees, Ph.D.

June 7, 2019

Paying attention to the specific measures required to protect equipment mounted on and contained within light poles used as small cell supports and enclosures saves airtime lost to outages and repair costs.

The next generation of millimeter-wave (mmW) 5G wireless communications technology deployment, will spur the use of short-range, small cell structures, mostly in the form of integrated street poles, in urban areas and cities.

These structures, often referred to as “smart” or “small cell” poles, usually comprise pole assemblies densely populated with electronic systems. The small cell sites can be built on existing or new metallic street lighting poles, either partially concealed or fully concealed, and on existing wooden utility poles. These electronic systems typically include:

Photo 1. Typical AC power and equipment compartments in an integrated 5G small cell pole.

In more sophisticated instances, these smart poles will also integrate smart city hubs containing sensors, such as high-resolution concealed cameras, gunshot detection microphones and atmospheric sensors for calculating the ultraviolet (UV) index and measuring solar brightness and solar radiation. In addition, the poles may accommodate additional structural subassemblies, such as support arms for LED street lighting, conventional sidewalk luminaries and receptacles for electric vehicle charging.

A centralized equipotential bonding system is usually provided within the pole via strategically positioned grounding bars, to which the different radio systems are connected. Typically, the neutral conductor of the incoming utility power supply also is bonded to ground at the energy meter’s socket, which in turn is bonded back to the main grounding bar. The pole’s external system ground is then bonded to this main grounding bar.

The simple light pole seen along sidewalks and city pavements is changing and will soon become a pivotal component of the new 5G wireless infrastructure. These systems will have paramount importance because they support the new technological layer of cellular networks for high-speed services. No longer will such pole structures simply accommodate incandescent light fixtures. Instead, they will become the core of a highly sophisticated technology. With this advance in integration, capability and reliance comes inevitable risk. Even with their relatively low heights compared to macro cell sites, such sophisticated electronic subsystems are set to become exponentially more susceptible to damage from overvoltage surges and transients.

Overvoltage Damage

Photo 2. Example of an AC power distribution enclosure with integrated overvoltage protection.

The importance of these small cells in the 5G infrastructure cannot be underestimated. Far from just being used to fill gaps in radio coverage and increase capacity, in 5G networks small cells will become the radio access network’s primary nodes, providing high-speed services in real time. These technologically advanced systems may well provide critical gigabit service links to customers where outages cannot be tolerated. This necessitates the use of highly reliable surge protection devices (SPDs) to maintain the availability of these sites.

The source of such overvoltage risks can broadly be categorized into two forms: those caused by radiated atmospheric disturbances and those caused by conducted electrical disturbances.

Let us consider each in turn:

Radiated disturbances are largely created by airborne events, such as nearby lightning discharges that create rapid changes in both electromagnetic and electrostatic fields around the structure. These rapidly varying electric and magnetic fields can couple with the electrical and electronic systems within the pole to produce damaging current and voltage surges. Indeed, the Faraday shielding created by the contiguous metallic structure of the pole will help reduce such effects; however, it cannot fully mitigate the problem. The sensitive antenna systems of these small cells are largely tuned to the frequencies at which much of the energy in the lightning discharge is centralized (5G will operate in frequency bands up to 39 GHz). Thus, they can act as conduits to allow this energy to enter the structure, causing possible damage to not only the radio front-ends, but also to other interconnected electronic systems within the pole.

Conducted disturbances are largely those that find their way into the pole via conductive cables. These include utility power conductors and signal lines, which can couple the internal electronic systems contained within the pole to the external environment. Because it is envisaged that the deployment of small cells will largely use the existing infrastructure of municipal street lightning or replace it with customized smart poles, small cells will rely on existing distribution wiring. Often, in the United States, such utility wiring is aerial and not buried. It is particularly susceptible to overvoltages, and a primary conduit for surge energy to enter the pole and damage the internal electronics.

Overvoltage protection (OVP)

Standards such as IEC 61643 describe the use of surge protective devices to mitigate the effects of such overvoltages. SPDs are classified by test class for the electrical environment within which they are intended to operate. For example, a Class I SPD is one that has been tested to withstand — using IEC terminology — “a direct or partial direct lightning discharge.” This means that the SPD has been tested to withstand the energy and waveform associated with the discharge most likely to enter a structure in an exposed location.

As we consider the deployment of small cell infrastructure, it is clear that the structures will be exposed. Many such poles are expected to appear along residential curbsides and pavements of metropolitan cities. It is also expected that such poles will proliferate in communal gathering places, such as indoor and outdoor sports stadiums, shopping centers and concert venues. Thus, it is important that the SPDs selected to protect the primary service entrance utility feed are suitably rated for this electrical environment and meet Class I testing, i.e., that they can withstand the energy associated with direct, or partially direct, lightning discharges. It is also recommended that the SPD selected have an impulse withstand level (Iimp) of 12.5 kA in order to safely withstand the threat level of such locations.

Selection of an SPD capable of withstanding the associated threat level is not in itself enough to ensure the equipment is afforded adequate protection. The SPD must also limit the incident conducted surge to a voltage protection level (Up) lower than the withstand level (Uw) of the electronic equipment within the pole. IEC recommends that Up< 0.8 Uw.

Raycap’s patented Strikesorb SPD technology is purposefully designed to provide the required Iimp and Up ratings to protect sensitive mission critical electronic equipment found in small cell infrastructures. Strikesorb technology is considered to be maintenance-free and can withstand thousands of repetitive surge events without failure or degradation. It provides a highly safe and reliable solution that eliminates the use of materials that could burn, smoke or explode. Based on years of field performance, Strikesorb’s expected lifetime is more than 20 years, and all modules are supplied with a 10-year limited lifetime warranty.

The products are tested according to international safety standards (UL and IEC) and offer unparalleled performance against lightning and power surges. Furthermore, Strikesorb protection is integrated into a compact AC distribution enclosure suitable to being installed within the small cell poles. This provides overcurrent protection to the incoming AC service and outgoing distribution circuits, thereby providing a convenient point at which the utility service from the electric meter can enter and distribute within the pole. These AC distribution enclosures are designed to meet the requirements of the National Electrical Code (NFPA 70) in order to be classified “suitable for use as service equipment” (SUSE) and are listed under UL 67.